The present invention relates to a shield for reducing the stray fields in the vicinity of a magnetic resonance magnets.
Magnetic resonance (MR) magnets used in medical diagnostics generate high intensity dc magnetic fields. Installations of MR equipment, where practicable, is located in a separate building in an open area. In hospitals located in crowded metropolitan areas where space is at a premium, it is desirable to have as little disturbance to other hospital facilities as possible. Without special shielding a 50 foot area surrounding a 1.5 Tesla MR magnet (1 Tesla=10 kilogauss) is needed to assure a minimum of interference with other diagnostic equipment and patients with cardiac pacemakers and neuro stimulators.
To reduce the space requirements, shielded rooms are erected surrounding the MR machines. The shielding has to be designed for the particular room shape and the shielding typically creates structural loading problems, since shielding can weigh 50 tons for a 1.5 T MR magnet.
Another concern with situating an MR magnet is the effect the surrouding structural environment has on the field homogeneity within the magnets working volume. A ferromagnetic body placed in the vicinity of an MR magnet will attract flux lines and this can be used to shield an external region from the stray field. However, the existence of the ferromagnetic body affects the distribution of flux lines throughout the space and will, therefore, affect the field homogeneity in the working volume. A nonuniform field in the working volume of the magnet is highly undesirable since it degrades the quality of the images produced by the MR machine.
It is an object of the present invention to provide an MR magnet shield to reduce the stray field using the least amount of ferromagnetic shield material.
It is a further object of the present invention to provide an MR magnet shield which uses the least amount of ferromagnetic shield material and at the same time minimizes the shield's effect on the field homogeneity in the working volume in the MR magnet.
It is a still further object of the present invention to provide an MR magnet shield suitable for retrofitting on the magnets of existing MR machines.
It is another object of the present invention to provide an MR magnet shield that desensitizes the magnet from the presence of ferromagnetic material in the vicinity of the MR magnet such as building structural supports.
It is another object of the present invention to reduce the attractive force on magnetizable object in the vicinity of the MR magnet, thereby reducing or eliminating the uncontrolled accelerating motion of these objects towards the MR magnet.